Burak Karan, 26, played in the youth teams with Germany’s current crop of stars including Sami Khedira and Kevin-Prince Boateng.
But in 2008 at the age of 20 he turned away from sport towards Islam, Focus magazine reported on Monday.
He travelled to Syria to join Islamist rebels fighting Syrian President Bashar Al-Assad where, his brother confirmed to Bild newspaper, he was killed by a bomb.
His 23-year-old wife and two children aged three and 10 months went with him to Syria, where they are believed still to be.
A video has appeared on YouTube portraying him as a engaged rebel soldier and claiming to be him, but his brother said that he was keen to dismiss this.
“It is in Arabic. Burak wanted to learn to speak the language perfectly but he couldn't," he told the Bild newspaper on Monday.
“He didn't want to fight,” he said. Regarding pictures of Karan holding a gun he said: "He was only armed to protect his vehicles.”
"Burak said to me that money and a career were not important to him," Mustafa said. He added that his brother would often search the internet for videos of war zones. "He was confused, and filled with sadness for the victims," he said.
The federal prosecutor's office has opened an investigation into whether Karan, from Wuppertal in North Rhine-Westphalia, was helping to support an overseas terror network.
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According to website transfermarket.de Karan played five times for the national U16 side and twice for the U17 team. On Monday, the website had changed its site, with “career ended” written on the 26-year-old's profile.